I currently have a dell XPS 420, 400 w psu, intel Q6600 2.4ghz processor, 3 gigs of ram, and 160 gigs of storage. I have a Radeon HD 3650. I don't like its overall performance, and would like something that can render the million polygon models in lightwave in less than 40 hours. I want the best graphics card a 400 watt psu can take, any card slot is ok pci-e, whatever, I just want a card that can handle the the visuals of causatics and radiosity, with particle effects added.
The best card a 400W PSU can take is the HD 5750 I'd say, if you want to be safe the HD 5670. Although I'm not sure how they would stand up against your tasks, you might want the FirePro versions instead.